Daniel-Constantin Mierla writes:
Grepping for that variable in the makefiles reveals that it looks for docbook2x-man or db2x_docbook2man.
Likely not available in your system, so either they were part of a installed package or related dependency in the past, or just a new package that needs to be added as build dependency.
In debian stretch, /usr/bin/docbook2x-man is in package docbook2x and that package is NOT listed as build dependency in pkg/kamailio/deb/stretch/control.
I added that package and the error was gone, but a new ones were produced:
/usr/bin/docbook2x-man -s ../../../doc/stylesheets/serdoc2man.xsl auth.xml auth.xml:7: warning: failed to load external entity "../../../../doc/docbook/entities.xml" %docentities; ^ Entity: line 1: %docentities; ^ auth.xml:25: parser error : Entity 'kamailio' not defined The <command>auth</command> module of &kamailio; provides basic functional ^ auth.xml:208: parser error : Entity 'kamailio' not defined can be divided into several partitions. Each &kamailio; process is assig ^ auth.xml:213: parser error : Entity 'kamailio' not defined conditions. In the worst case, when only one &kamailio; process receive ^ auth.xml:532: parser error : Entity 'kamailio' not defined separated by a comma. For challenges generated by &kamailio;, this ^ auth.xml:535: parser error : Entity 'kamailio' not defined parameter will be present in challenges created by &kamailio;. This may ^ auth.xml:537: parser error : Entity 'kamailio' not defined make your &kamailio; not comply with RFC 3261. ^ -- Juha